Tunable LED-Filaments and Tunable LED-Filament Lamps

ABSTRACT

An LED-filament includes a light-transmissive substrate; a first array of LED chips on a front face of the substrate; a second array of LED chips on the front face of the substrate; a first photoluminescence arrangement covering the first array of LED chips; and a second photoluminescence arrangement covering the second array of LED chips; where the first array of LED chips and the first arrangement generate light of a first color temperature and the second array of LED chips and the second arrangement generate light of a second color temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to international PCTapplication PCT/US20/23187, filed Mar. 17, 2020, of which thisapplication is a bypass continuation-in-part; which in turn claimspriority to (a) U.S. provisional patent application No. 62/820,249,filed Mar. 18, 2019, (b) U.S. provisional patent application No.62/831,699, filed Apr. 9, 2019 and (c) U.S. utility patent applicationSer. No. 16/540,019, filed Aug. 13, 2019, (d) U.S. provisional patentapplication No. 62/941,735, filed Nov. 28, 2019, and (e) U.S.provisional patent application No. 62/941,734, filed Nov. 28, 2019, allof which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

Embodiments of the present invention are directed to tunableLED-filaments and tunable LED-filament lamps.

BACKGROUND OF THE INVENTION

White light emitting LEDs (“white LEDs”) include one or morephotoluminescence materials (typically inorganic phosphor materials),which absorb a portion of the blue light emitted by the LED and re-emitlight of a different color (wavelength). The portion of the blue lightgenerated by the LED that is not absorbed by the phosphor materialcombined with the light emitted by the phosphor provides light whichappears to the eye as being white in color. Due to their long operatinglife expectancy (>50,000 hours) and high luminous efficacy (1001 m/W andhigher), white LEDs are rapidly being used to replace conventionalfluorescent, compact fluorescent and incandescent lamps.

Recently, LED-filament lamps have been developed comprisingLED-filaments whose visual appearance closely resembles the filament ofa traditional incandescent lamp. Such an appearance is highly desirableto consumers and sales of LED-filament lamps have grown rapidly. TheLED-filaments, which are typically about 2 inches (52 mm) long, compriseCOG (Chip-On-Glass) or COC (Chip-On-Ceramic) devices having a pluralityof low-power LED chips mounted on one face of a light-transmissive(transparent) glass or ceramic substrate. Front and back faces of thelight-transmissive substrate are coated with a phosphor-loadedencapsulant, such as silicone. Typically, the phosphor comprises amixture of green and red light emitting phosphors for generating warmwhite light and to increase General Color Rendering Index (CRI Ra) oflight generated by the filament. The same phosphor-loaded encapsulant isapplied to both faces of the substrate to ensure that the filamentgenerates the same color of light in forward and backward directions.

A perceived shortcoming or characteristic of white LEDs, especiallyLED-filaments and LED-filament lamps, compared with traditionalincandescent bulbs, is that as they are dimmed (i.e. the intensity ofgenerated light decreased by decreasing power to the device) the colortemperature (CCT—Correlated Color Temperature) of their light emissiondoes not change; that is, it remains substantially constant and only theintensity varies. By contrast—for an incandescent lamp—the CCT canchange from a warm white (2700K) at full power to a warm orange glow(900K) when fully dimmed; a range of 1800K. The property of a lightemitting device whose color temperature of emitted light decreases ondimming is referred to “warm dimming”. Warm dimming would be highlydesirable for many applications, especially for LED-filaments. Toimplement an LED-filament that exhibits warm dimming requirescolor/color temperature tunable LED-filaments.

Color temperature tunable white LED lamps, for example LED bulbs anddownlights, are known that comprise a combination of “cool white” and“warm white” packaged white LEDs enabling tuning between cool white andwarm white and CCTs in between. To achieve a uniform color of emittedlight requires adequate mixing of the light of different colortemperatures and LED bulbs/downlights include a diffuser. In contrast,for an LED-filament lamp that includes LED-filaments of different CCTsthe LED-filaments are spaced apart too far to achieve adequate mixing.While it is possible that the bulb envelope could include a diffuser,such a feature is undesirable since it would prevent viewing of theLED-filaments that reduce visual appeal of LED-the filament lamp.Without a diffuser the different color temperature LED-filaments wouldbe clearly visible and this would further degrade the appearance of thelamp.

It is an object of the present invention to provide a tunableLED-filament that at least in part addresses the limitations of knownLED-filaments and whose emitted light is color/color temperaturetunable. Moreover, it is desirable to provide an LED-filament thatexhibits warm dimming.

SUMMARY OF THE INVENTION

Embodiments of the invention concern tunable LED-filaments comprisingtwo arrays of LED chips (LED arrays) on the same substrate that areconfigured, through the use of photoluminescence material arrangements,to generate light of two different colors/color temperatures.

In accordance with embodiments of the invention an LED-filamentcomprises: a light-transmissive substrate; a first array of LED chips ona front face of the substrate; a second array of LED chips on the frontface of the substrate; a first photoluminescence arrangement coveringthe first array of LED chips; and a second photoluminescence arrangementcovering the second array of LED chips; wherein the first array of LEDchips and the first arrangement generate light of a first colortemperature and the second array of LED chips and the second arrangementgenerate light of a second color temperature.

It may be that the first color temperature is 1500K to 3500K and thesecond color temperature is 3500K to 7500K. Such a range of colortemperature tuning is highly desirable for general lighting.

Alternatively it may be the first color temperature is 1800K to 2500K,and the second color temperature is 3000K to 4000K. Such a range ofcolor temperature tuning is highly desirable for “warm dimming”.

In one embodiment the first arrangement comprises a first layer having afirst photoluminescence material disposed on the first array of LEDchips and the second arrangement comprises a second layer having asecond photoluminescence material disposed on the second array of LEDchips. A particular advantage of such an LED-filament comprising onlytwo photoluminescence layers to generate the first and second colortemperatures of light is ease of fabrication. Since the LED-filament ismanufactured with the first and second array of LED chips mounted on thesame substrate, this enhances the robustness of the LED-filament andreduces the likelihood of error during manufacturing since there arefewer parts to be assembled compared with an LED-filament having, forexample, two substrates each having its own respective array of LEDchips.

The LED-filament can comprise a diffusing layer comprising particles oflight scattering material wherein the diffusing layer is disposed on thefirst and second layers. Such a diffusing layer can improve mixing oflight of the first and second color temperatures improving the coloruniformity of the light generated by the LED-filament.

It may be that the first photoluminescence material comprises a redphotoluminescence material and the first arrangement further comprises athird layer having a third photoluminescence material disposed on thefirst layer. A particular advantage of locating the redphotoluminescence material in a separate respective layer is that thiscan reduce the usage of such materials. This can provide a substantialcost saving when using expensive narrowband red photoluminescencematerials such as K₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺, and K₂TiF₆:Mn⁴⁺.

Similarly, the second photoluminescence material may comprise a redphotoluminescence material and the second arrangement further comprisesa fourth layer having a fourth photoluminescence material disposed onthe second layer. Again, a particular advantage of locating the redphotoluminescence material in a separate respective layer is that thiscan reduce the usage of such materials. This can provide a substantialcost saving when using expensive narrowband red photoluminescencematerials such as K₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺, and K₂TiF₆:Mn⁴⁺.

Embodiments in which the third and fourth layers comprise redphotoluminescence materials can find particular utility when light ofthe first and second color temperatures are similar (<1500K difference),especially lower (warmer), color temperatures, e.g. “warm white” to“amber” and cooler “warm white” thereby enabling the LED-filament to betunable between “amber” and “warm white”. Such a range of color tuningis highly desirable for “warm dimming”.

To ensure the front and back faces of the LED-filament generate light ofsimilar color temperatures the LED-filament may comprise a fifth layerhaving a fifth photoluminescence material disposed on a back face of thesubstrate and positionally in correspondence with the first array of LEDchips, and a sixth layer comprising a sixth photoluminescence materialdisposed on a back face of the substrate and positionally incorrespondence with the second array of LED chips. The fifthphotoluminescence materials can be configured such that the LED-filamentemits light from its back face of the first color temperature and thesixth photoluminescence can be configured such that the LED-filamentemits light from its back face of the second color temperature.

In other embodiments the LED-filament may comprise: a first layer havinga first photoluminescence material disposed on the first array of LEDchips and a second layer having a second photoluminescence materialdisposed on the first layer and the second array of LED chips, whereinthe first arrangement comprises the first and second firstphotoluminescence materials and the second arrangement comprisessubstantially the second photoluminescence material. In such arrangementthe second layer covers both the first and second arrays of LED chipsand light of the first color temperature is generated by the combinationof the first and second photoluminescence materials of the first andsecond layers and light of the second color temperature is generated bythe first photoluminescence material. A particular advantage of such anLED-filament comprising only two photoluminescence layers to generatethe first and second color temperatures of light is ease of fabrication.Moreover, since the second layer covers the first layer this can improvemixing of light and improve color uniformity of emitted light. In suchan arrangement, it may be that the first layer comprises a majority oforange to red photoluminescence material.

To ensure the front and back faces of the LED-filament generate light ofsimilar color temperatures the LED-filament may comprise a third layerhaving a third photoluminescence material disposed on the back face ofthe substrate. The third photoluminescence material can be configured togenerate light of a color temperature that is between the first andsecond color temperature when both arrays of LED chips are activated.

In any embodiment at least one of said layers comprises particles of alight scattering material. The inclusion of light scattering particlescan reduce photoluminescence material usage and improve light coloruniformity.

In any embodiment at least one of said photoluminescence materials cancomprise green to yellow photoluminescence materials, narrowband redphotoluminescence materials, broadband orange to red photoluminescencematerials and combinations thereof

The narrowband red photoluminescence material can be at least one of:K₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺, and K₂TiF₆:Mn⁴⁺. The broadband orange to redphotoluminescence material can comprise a rare-earth-activated redphotoluminescence material. The rare-earth-activated orange to redphotoluminescence material can be at least one of: a nitride-basedphosphor material having a general composition AAlSiN₃: Eu²⁺ where A isat least one of Ca, Sr or Ba; a sulfur-based phosphor material having ageneral composition (Ca_(1-x)Sr_(x))(Se_(1-y)S_(y)):Eu²⁺ where 0≤x≤1 and0<y≤1; and a silicate-based phosphor material having a generalcomposition (Ba_(1-x)Sr_(x))₃SiO₅:Eu²⁺ where 0≤x≤1.

The orange to red photoluminescence material can comprise a narrowbandred photoluminescence material or a broadband red photoluminescencematerial and combinations thereof. The narrowband red photoluminescencematerial can be at least one of: K₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺, andK₂TiF₆:Mn⁴⁺. The broadband red photoluminescence material can comprise arare-earth-activated red photoluminescence material. Therare-earth-activated orange to red photoluminescence material can be atleast one of: a nitride-based phosphor material having a generalcomposition AAlSiN₃: Eu²⁺ where A is at least one of Ca, Sr or Ba; asulfur-based phosphor material having a general composition(Ca_(1-x)Sr_(x))(Se_(1-y)S_(y)):Eu²⁺ where 0≤x≤1 and 0<y≤1; and asilicate-based phosphor material having a general composition(Ba_(1-x)Sr_(x))₃SiO₅:Eu²⁺ where 0≤x≤1.

In embodiments, the first and second arrays of LED chips eachrespectively comprise a linear array of serially electrically connectedLED chips. The LED-filament may further comprise a resistive componentconnected in series with the first array of LED chips and wherein thefirst and second arrays are electrically connected in parallel. It maybe that the first array of LED chips comprises fewer LED chips than thesecond array of LED chip. Such a circuit arrangement allows theLED-filament to exhibit “warm dimming”.

It may be that the substrate has a transmittance of from 2% to 70%.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention inconjunction with the accompanying figures, in which:

FIGS. 1A and 1B respectively illustrate partial cross-sectional A-A sideand plan views of a four LED-filament A-Series (A19) lamp in accordancewith an embodiment of the invention;

FIGS. 2A and 2B respectively illustrate schematic cross-sectional sideB-B and partial cutaway plan of color temperature tunable LED-filamentsin accordance with embodiments of the invention for use in the lamp ofFIGS. 1A and 1B;

FIGS. 3 and 4 are schematic cross-sectional end views C-C of colortemperature tunable LED-filaments in accordance with embodiments of theinvention comprising “single-layer” photoluminescence arrangements;

FIG. 5 is a schematic cross-sectional end view C-C of a colortemperature tunable LED-filament in accordance with an embodiment of theinvention comprising “double-layer” photoluminescence arrangements;

FIG. 6 is a schematic cross-sectional end view C-C of a colortemperature tunable LED-filament in accordance with an embodiment of theinvention comprising “single-layer” and “double-layer” photoluminescencearrangements;

FIG. 7A is a schematic of a driver circuit arrangement for use with thecolor temperature tunable LED-filaments of FIGS. 3 to 6 with independentpower control to the first and second LED arrays;

FIG. 7B is a schematic of a second driver circuit arrangement for usewith the color temperature tunable LED-filaments of FIGS. 3 to 6 thatexhibits “warm dimming”;

FIGS. 8A to 8C are test data for a first “warm dimming” LED-filament inaccordance with FIG. 7B and respectively show: (a) current/voltagecharacteristic of the first and second LED arrays of the LED-filament,(b) currents I_(a) and I_(b) (mA) flowing through the first and secondLED arrays as a function of filament power (W), and (c) current ratioI_(x)/I (%) for currents I_(a) and I_(b) flowing through the first andsecond LED arrays as a function of filament power (W);

FIGS. 9A to 9C are test data for a second “warm dimming” LED-filament inaccordance with FIG. 7B and respectively show: (a) current/voltagecharacteristic of the first and second LED arrays of the LED-filament,(a) currents I_(a) and I_(b) (mA) flowing through the first and secondLED arrays as a function of filament power (W), and (c) current ratioI_(x)/I (%) for currents I_(a) and I_(b) flowing through the first andsecond LED arrays as a function of filament power (W); and

FIGS. 10A and 10B are measured test data for a tunable LED-filament withthe photoluminescence arrangement of FIG. 6 comprising a first LED arrayfor generating light of CCT 2700K and a second LED array for generatinglight of CCT 5000K.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples of the invention so as to enable those skilled in the art topractice the invention. Notably, the figures and examples below are notmeant to limit the scope of the present invention to a singleembodiment, but other embodiments are possible by way of interchange ofsome or all of the described or illustrated elements. Moreover, wherecertain elements of the present invention can be partially or fullyimplemented using known components, only those portions of such knowncomponents that are necessary for an understanding of the presentinvention will be described, and detailed descriptions of other portionsof such known components will be omitted so as not to obscure theinvention. In the present specification, an embodiment showing asingular component should not be considered limiting; rather, theinvention is intended to encompass other embodiments including aplurality of the same component, and vice-versa, unless explicitlystated otherwise herein. Moreover, applicants do not intend for any termin the specification or claims to be ascribed an uncommon or specialmeaning unless explicitly set forth as such. Further, the presentinvention encompasses present and future known equivalents to the knowncomponents referred to herein by way of illustration. Throughout thisspecification, like reference numerals preceded by the figure number areused to denote like parts.

FIGS. 1A and 1B respectively illustrate a partial cross-sectional sideview through A-A and a partial cutaway plan view of an LED-filamentA-Series lamp (bulb) 100 formed in accordance with an embodiment of theinvention. The LED-filament lamp (bulb) 100 is intended to be an energyefficient replacement for a traditional incandescent A19 light bulb andcan be configured to generate 550 lm of light with a CCT of 2700 K and ageneral color rendering index CRI Ra of at least 80. The LED-filamentlamp is nominally rated at 4W. As is known, an A-series lamp is the mostcommon lamp type and an A19 lamp is 2 ⅜ inches ( 19/8 inches) wide atits widest point and approximately 4 ⅜ inches in length.

The LED-filament lamp 100 comprises a connector base 102, alight-transmissive envelope 104; an LED-filament support 106 and fourtunable LED-filaments 108 _(iv), 108 _(ii), 108 _(iii), 108 _(iv).

In some embodiments, the LED-filament lamp 100 can be configured foroperation with a 110V (r.m.s.) AC (60 Hz) mains power supply as used inNorth America. For example and as illustrated, the LED-filament lamp 100can comprise an E26 (ϕ26 mm) connector base (Edison screw lamp base) 102enabling the lamp to be directly connected to a mains power supply usinga standard electrical lighting screw socket. It will be appreciated thatdepending on the intended application other connector bases can be usedsuch as, for example, a double contact bayonet connector (i.e. B22d orBC) as is commonly used in the United Kingdom, Ireland, Australia, NewZealand and various parts of the British Commonwealth or an E27 (ϕ27 mm)screw base (Edison screw lamp base) as used in Europe. The connectorbase 102 can house rectifier or other driver circuitry (not shown) foroperating the LED-filament lamp.

The light-transmissive envelope 104 is attached to the connector 102.The light-transmissive envelope 104 and LED-filament support 106 cancomprise glass. The envelope 104 defines a hermetically sealed volume110 in which the LED-filaments 108 _(i) to 108 _(iv) are located. Theenvelope 104 may additionally incorporate or include a layer of a lightdiffusive (scattering) material such as for example particles of zincoxide (ZnO), titanium dioxide (TiO₂), barium sulfate (BaSO₄), magnesiumoxide (MgO), silicon dioxide (SiO₂) or aluminum oxide (Al₂O₃).

The LED-filaments 108 _(i) to 108 _(iv), which are linear (strip orelongate) in form, are oriented such that their direction of elongationis generally parallel to an axis 112 of the lamp 100. In thisembodiment, the LED-filaments 108 _(i) to 108 _(iv) are equallycircumferentially spaced around the glass filament support 106 (FIG.1B), although it will be appreciated that in other embodiments theLED-filaments may not be equally spaced around the glass support. Afirst electrical contact 114 _(i) to 114 _(iv) on a first end of eachrespective LED-filament 108 _(i) to 108 _(iv) distal to the connectorbase 102 is electrically and mechanically connected to a firstconducting wire 116 that passes down an axis of the LED filament support106 to the connector base 102. A second electrical contact 118 _(i) to118 _(iv) on a second end of each LED-filament 108 _(i) to 108 _(iv)proximal to the connector base 102 is electrically and mechanicallyconnected to a second conducting wire 120 that passes through a baseportion 122 of the LED filament support 106 to the connector base 102.As illustrated, the LED filaments 108 _(i) to 108 _(iv) can beelectrically connected in parallel.

As described above, tunable LED-filaments in accordance with embodimentsof the invention comprise two arrays of LED chips (LED arrays) on thesame substrate that are configured, through the use of photoluminescencematerials of various structures, to generate light of two differentcolors/color temperatures. In this specification, the first array ofLEDs generates light of a first color temperature and the second arrayof LED chips generates light of a second higher color temperature (i.e.cooler color).

A tunable LED-filament according to an embodiment of the invention isnow described with reference to FIGS. 2A and 2B which respectively showa cross-sectional side view through B-B, a partial cut-away plan and apartial cutaway plan view of a tunable LED-filament 208 in accordanceembodiments of the invention.

The LED-filament 208 comprises a light-transmissive substrate 224 havingfirst and second arrays (plurality) of blue emitting (465 nm) unpackagedLED chips (dies) 226 a, 226 b mounted directly to a front (first) face228. For the sake of brevity, the first and second arrays of LED chipswill respectively be referred to as first and second LED arrays. In thisspecification, the suffixes a and b are used to indicate the LED arrayand features (e.g. photoluminescence material layers, contacts, etc.)respectively associated with the array of LED chips. Typically, eachLED-filament has a total nominal power of about 0.7 W to 1 W.

The substrate 224 can further comprise respective electrical first andsecond contacts 214 a, 214 b, 218 a, 218 b on the front face 228 at thefirst and second ends of the substrate 224 for electrical connection toa respective one of the conducting wires 116, 120 (FIG. 1A) to provideelectrical power to operate the LED-filament. The electrical contacts214 a, 214 b, 218 a, 218 b can comprise copper, silver or other metal ora transparent electrical conductor such as indium tin oxide (ITO). Inthe embodiment, illustrated the substrate 224 is planar and has anelongate form (strip) with the arrays of LED chips 226 a, 226 b areconfigured as linear arrays (string) and equally spaced along the length(direction of elongation) of the substrate. As indicated in FIGS. 2A and2B the LED chips 226 a, 226 b can be electrically connected in series byrespective bond wires 230 a, 230 b between adjacent the LED chips of thestring and bond wires 232 a, 232 b between the LED chips at the distalends of the substrate and their respective electrical contact 214 a, 214b, 218 a, 218 b.

When the LED-filament 208 is used as a part of an energy efficient bulb,an elongate configuration is typically preferred since the appearanceand emission characteristics of the device more closely resembles atraditional filament of an incandescent bulb. It should be noted thatthe LED chips 226 a, 226 b are unpackaged and emit light from both theirtop and bottom (base) faces with the base surface of the LED chipmounted directly on the substrate 224.

The light-transmissive substrate 224 can comprise any material which islight-transmissive and can have a transmittance to visible light from 2%to 70% (reflectance of 98% to 30%). The substrate can comprise a glass,ceramic material or a plastics material such as polypropylene, siliconeor an acrylic. In embodiments, the light-transmissive substrate cancomprise a porous ceramic substrate composed of alumina that has atransmittance of about 40%. To aid in the dissipation of heat generatedby the LED chips 226 a, 226 b, the substrate 224 can not only belight-transmissive, but can also be thermally conductive to aid in thedissipation of heat generated by the LED chips. Examples of suitablelight-transmissive thermally conductive materials include: magnesiumoxide, sapphire, aluminum oxide, quartz glass, and diamond. Thetransmittance of the thermally conductive substrate can be increased bymaking the substrate thin. To increase mechanical strength, thesubstrate can comprise a laminated structure with the thermallyconductive layer mounted on a light-transmissive support such as a glassor plastics material. To further assist in the dissipation of heat, thevolume 110 (FIG. 1A) within the glass envelope 104 (FIG. 1A) can befilled with a thermally conductive gas such as helium, hydrogen or amixture thereof.

The LED-filament 208 further comprises a photoluminescence wavelengthconversion coating (layer) 236 applied to/disposed on and covering thefront face 228 of the substrate 224 and may optionally comprise aphotoluminescence wavelength conversion coating 238 applied to andcovering/disposed on the second/back (opposite) face 234 of thesubstrate 224.

In accordance with embodiments of the invention the photoluminescencecoating 236 comprises a first photoluminescence material arrangement(structure) that covers the first LED array 226 a and a secondphotoluminescence material arrangement that covers the second LED array226 b. The first array of LED chips in combination with the firstphotoluminescence arrangement are configured to generate light of afirst color temperature and the second array of LED chips in combinationwith the second photoluminescence arrangement are configured to generatelight of a second cooler color temperature. The photoluminescenceconversion materials can comprise one or more green to yellowphotoluminescence materials having a peak emission wavelength rangingfrom 520 nm to 560 nm (preferably 540 nm to 545 nm), one or more orangeto red photoluminescence materials having a peak emission wavelengthranging from 620 nm to 650 nm and combinations thereof. Suitable greento yellow photoluminescence materials and orange to redphotoluminescence materials are discussed below.

Various photoluminescence arrangements (structures) for the front andback faces of the substrate are now described by of reference to FIGS. 3to 6.

FIG. 3 is a schematic cross-sectional end view C-C of a colortemperature tunable LED-filament 308 in accordance with an embodiment ofthe invention comprising “single-layer” photoluminescence arrangements.In the embodiment illustrated in FIG. 3, the first photoluminescencearrangement covering the first array of LED chips 326 a comprises: aphotoluminescence layer 340 comprising a first photoluminescencematerial disposed on the first LED array 326 a and the secondphotoluminescence arrangement covering the second array of LED chips 326b comprises: a photoluminescence layer 342 comprising a secondphotoluminescence material disposed on the second LED array 326 b.

Since the first and second photoluminescence arrangements each comprisea single photoluminescence layer such structures will be referred to asa single-layer structure.

The photoluminescence coating 338 covering the back face 334 of thesubstrate 324 comprises a photoluminescence layer 344 comprising a thirdphotoluminescence material and a photoluminescence layer 346 comprisinga fourth photoluminescence material. As illustrated, thephotoluminescence layer 344, containing the third photoluminescencematerial, is disposed on and covers a part of the substrate 324positionally in correspondence with the first LED array 326 a and thephotoluminescence layer 346, containing the fourth photoluminescencematerial, is disposed on and covers a part of the substrate 324positionally in correspondence with the second LED array 326 b.

The first and third photoluminescence materials are selected such thatwhen excited by blue light from the first LED array 326 a they generategreen to red light which in combination with unconverted blue excitationlight generates a combined light emission 348 of the first colortemperature. Typically the first and third photoluminescence materialswill be compositionally very similar.

The second and fourth photoluminescence materials are selected such thatwhen excited by blue light from the second LED array 326 b they generategreen to red light which in combination with unconverted blue excitationlight generates a combined light emission 350 of the second cooler colortemperature. Typically the second and fourth photoluminescence materialswill be compositionally very similar.

It will be appreciated that depending on the relative proportion oflight generated by the first and second LED arrays the color temperatureof the LED-filament can be tuned between the first and second colortemperatures. Additionally, particles of a light scattering material canbe combined with the photoluminescence materials to reduce the quantityof photoluminescence material required to generate a given emissionproduct color.

The single-layer LED-filament of FIG. 3 can be manufactured bydepositing the photoluminescence layer 340 onto the first array of LEDchips 326 a and then depositing the photoluminescence layer 342 onto thesecond array of LED chips 326 b. Next, the photoluminescence layer 344can be deposited the back face 334 of the substrate 324 to cover atleast the part of the substrate 324 positionally in correspondence withthe first LED array 326 a and the photoluminescence layer 346 depositedon the back face 334 of the substrate 324 to cover at least the part ofthe substrate 324 positionally in correspondence with the second LEDarray 326 b. A particular advantage of the structure of thephotoluminescence structure of FIG. 3 is its ease of fabrication. Asillustrated, the photoluminescence layers 340, 342, 344, 346 can have across section that is generally semi-circular in profile.

The LED-filament of FIG. 3 can find particular utility where it isrequired that the first and second LED arrays generate light ofsubstantially different color temperatures (>1500K difference), e.g.“warm white” and “cool white” thereby enabling the LED-filament to betunable between “warm white” and “cool white”. For example, thephotoluminescence composition of the photoluminescence layer 340 canconfigured such that the first LED array 326 a in conjunction with thephotoluminescence layer 340 generates white light of a first colortemperature, for example “warm white” (e.g. CCT 1500K to 3500K), whilethe second LED array 326 b in conjunction with the photoluminescencelayer 342 generates white light of a second, higher, color temperature,for example “cool white” (e.g. CCT 3500K to 7500K).

FIG. 4 is a schematic cross-sectional end view C-C of a colortemperature tunable LED-filament 408 in accordance with an embodiment ofthe invention comprising “single-layer” photoluminescence arrangements.As with embodiment of FIG. 3, the first photoluminescence arrangementcovering the first array of LED chips 426 a comprises aphotoluminescence layer 440 comprising a first photoluminescencematerial disposed on the first LED array 426 a and the secondphotoluminescence arrangement covering the second array of LED chips 426b comprises a photoluminescence layer 442 comprising a secondphotoluminescence material disposed on the second LED array 426 b. Inthis embodiment the LED-filament further comprises a light diffusinglayer 452 comprising particles of light scattering material that isdisposed on and covers the first and second layers 440, 442.

The photoluminescence coating 438 covering the back face 434 of thesubstrate 424 comprises a photoluminescence layer 444 comprising a thirdphotoluminescence material and a photoluminescence layer 446 comprisinga fourth photoluminescence material. As illustrated, thephotoluminescence layer 444, containing the third photoluminescencematerial, is disposed on and covers a part of the substrate 424positionally in correspondence with the first LED array 426 a and thephotoluminescence layer 446, containing the fourth photoluminescencematerial, is disposed on and covers a part of the substrate 424positionally in correspondence with the second LED array 426 b.

The first and third photoluminescence materials are selected such thatwhen excited by blue light from the first LED array 426 a they generategreen to red light which in combination with unconverted blue excitationlight generates a combined light emission 448 of the first colortemperature. Typically the first and third photoluminescence materialswill be compositionally very similar.

The second and fourth photoluminescence materials are selected such thatwhen excited by blue light from the second LED array 426 b they generategreen to red light which in combination with unconverted blue excitationlight generates a combined light emission 450 of the second cooler colortemperature. Typically the second and fourth photoluminescence materialswill be compositionally very similar.

It will be appreciated that depending on the relative proportion oflight generated by the first and second LED arrays the color temperatureof the LED-filament can be tuned between the first and second colortemperatures. Additionally, particles of a light scattering material canbe combined with the photoluminescence materials to reduce the quantityof photoluminescence material required to generate a given emissionproduct color.

The single-layer LED-filament of FIG. 4 can be manufactured bydepositing the photoluminescence layer 440 onto the first array of LEDchips 426 a, depositing the photoluminescence layer 442 onto the secondarray of LED chips 426 b, and then depositing the light diffusing layer452 onto to the first and second layers 440, 442. Next, thephotoluminescence layer 444 can be deposited the back face 434 of thesubstrate 424 to cover at least the part of the substrate 424positionally in correspondence with the first LED array 426 a and thephotoluminescence layer 446 deposited on the back face 434 of thesubstrate 424 to cover at least the part of the substrate 424positionally in correspondence with the second LED array 426 b. Aparticular advantage of the structure of the photoluminescence structureof FIG. 4 is its ease of fabrication. As illustrated, thephotoluminescence layers 440, 442, 444, 446 can have a cross sectionthat is generally semi-circular in profile.

FIG. 5 is a schematic cross-sectional end view C-C of a colortemperature tunable LED-filament 508 in accordance with an embodiment ofthe invention comprising a “double-layer” photoluminescence arrangement.In the embodiment illustrated in FIG. 5, the first and secondphotoluminescence arrangements each comprise two layer, “double-layer”structure, covering their respective LED array. The firstphotoluminescence arrangement comprises: a photoluminescence layer 540comprising a first photoluminescence material disposed on the first LEDarray 526 a and a photoluminescence layer 554 comprising a secondphotoluminescence material disposed on and covering thephotoluminescence layer 540. The second photoluminescence arrangementcomprises: a photoluminescence layer 542 comprising a thirdphotoluminescence material disposed on the second LED array 526 b and aphotoluminescence layer 556 comprising a fourth photoluminescencematerial disposed on and covering the photoluminescence layer 542 theLED array 526 a and while the second photoluminescence arrangementcovering the second array of LED chips 326 b comprises a secondphotoluminescence layer 342.

The photoluminescence coating 538 covering the back face 534 of thesubstrate 524 comprises a photoluminescence layer 544 comprising a fifthphotoluminescence material and a photoluminescence layer 546 comprisinga sixth photoluminescence material. As illustrated, thephotoluminescence layer 544, containing the fifth photoluminescencematerial, is disposed on and covers a part of the substrate 524positionally in correspondence with the first LED array 526 a and thephotoluminescence layer 546, containing the sixth photoluminescencematerial, is disposed on and covers a part of the substrate 524positionally in correspondence with the second LED array 426 b.

The first and second photoluminescence materials on the front face ofthe LED-filament are selected such that when they are excited by bluelight from the first LED array 526 a they generate green to red lightwhich in combination with unconverted blue excitation light generates acombined light emission 548 of the first color temperature. Typically,the first photoluminescence material comprises a red photoluminescence.A particular advantage of locating the red photoluminescence material ina separate respective layer 540, rather than as a mixture with thesecond photoluminescence in a single layer, is that this can reduce theusage of such materials. This can provide a substantial cost saving whenusing expensive narrowband red photoluminescence materials such asK₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺, and K₂TiF₆:Mn⁴⁺. The fifth photoluminescencematerial on the back face of the LED-filament is selected such that whenexcited by blue light from the first LED array 526 a it generate greento red light which in combination with unconverted blue excitation lightgenerates a combined light emission 548 of the first color temperature.

The third and fourth photoluminescence materials on the front face ofthe LED-filament are selected such that when they are excited by bluelight from the second LED array 526 b they generate green to red lightwhich in combination with unconverted blue excitation light generates acombined light emission 550 of the second color temperature. Typically,the third photoluminescence material comprises a red photoluminescence.A particular advantage of locating the red photoluminescence material ina separate respective layer 542, rather than as a mixture with thefourth photoluminescence in a single layer, is that this can reduce theusage of such materials. This can provide a substantial cost saving whenusing expensive narrowband red photoluminescence materials such asK₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺, and K₂TiF₆:Mn⁴⁺. The sixth photoluminescencematerial on the back face of the LED-filament is selected such that whenexcited by blue light from the second LED array 526 b it generate greento red light which in combination with unconverted blue excitation lightgenerates a combined light emission 550 of the second color temperature.

It will be appreciated that depending on the relative proportion oflight generated by the first and second LED arrays the color temperatureof the LED-filament can be tuned between the first and second colortemperatures. Additionally, particles of a light scattering material canbe combined with the photoluminescence materials to reduce the quantityof photoluminescence material required to generate a given emissionproduct color.

The double-layer LED-filament of FIG. 5 can be manufactured bydepositing the photoluminescence layer 540 onto the first array of LEDchips 526 a, depositing the photoluminescence layer 554 onto thephotoluminescence layer 540, depositing the photoluminescence layer 542onto the second array of LED chips 326 b, and depositing thephotoluminescence layer 556 onto the photoluminescence layer 542. Next,the photoluminescence layer 544 can be deposited the back face 534 ofthe substrate 524 to cover at least the part of the substrate 524positionally in correspondence with the first LED array 526 a and thephotoluminescence layer 546 deposited on the back face 534 of thesubstrate 524 to cover at least the part of the substrate 524positionally in correspondence with the second LED array 526 b. Asillustrated, the photoluminescence layers 540, 542, 544, 546, 554, 556can have a cross section that is generally semi-circular in profile.

The LED-filament of FIG. 5 can find particular utility where it isrequired that the first and second LED arrays generate light ofsubstantially different color temperatures (>1500K difference), e.g.“warm white” and “cool white” thereby enabling the LED-filament to betunable between “warm white” and “cool white”. For example, thephotoluminescence composition of the photoluminescence layers 540 and554 can be configured such that the first LED array 526 a in conjunctionwith the photoluminescence layers 540, 554 generates white light of afirst color temperature, for example “warm white” (e.g. CCT 1800K to2500K), while the second LED array 526 b in conjunction with thephotoluminescence layers 542, 556 generates white light of a second,higher, color temperature, for example “cool white” (e.g. CCT 3000K to4000K). In one such LED-filament the photoluminescence layers 540, 542can comprise, for example, a majority orange to red photoluminescencematerials such as KSF and/or CASN and the photoluminescence layer 554can comprise a mixture of GNYAG and optionally an orange to redphotoluminescence material.

FIG. 6 is a schematic cross-sectional end view C-C of a colortemperature tunable LED-filament 608 in accordance with an embodiment ofthe invention comprising “double-layer” and “single-layer”photoluminescence arrangements. In the embodiment illustrated in FIG. 6,the first photoluminescence arrangement covering the first LED arraycomprises a “double-layer” structure, while second photoluminescencearrangement covering the second LED array comprises a “single-layer”structure.

In the embodiment of FIG. 6, a photoluminescence layer 640 comprising afirst photoluminescence material is disposed on the first LED array 626a and a photoluminescence layer 642 comprising a secondphotoluminescence material is disposed on, and covers both thephotoluminescence layer 640 and the second LED array 626 b. In thisembodiment the first photoluminescence arrangement is constituted by thecombination of the photoluminescence layers 640, 642 and consequentlycomprises a “double-layer” structure. The second photoluminescencearrangement is constituted by the photoluminescence layer 642 alone andconsequently comprises a “single-layer”.

The back face 634 of the substrate 624 can be covered with aphotoluminescence layer 658 comprising a third photoluminescencematerial.

The first and second photoluminescence materials on the front face ofthe LED-filament are selected such that when they are excited by bluelight from the first LED array 626 a only they generate green to redlight which in combination with unconverted blue excitation lightgenerates a combined light emission 648 of the first color temperature.The first photoluminescence material may comprise a majority of anorange to red photoluminescence.

The second photoluminescence material on the front face of theLED-filament is selected such that when it excited by blue light fromthe second LED array 626 b only it generates green to red light which incombination with unconverted blue excitation light generates a combinedlight emission 650 of the second color temperature.

The third photoluminescence material on the back face of theLED-filament is selected such that when it excited by blue light fromthe first and second LED arrays 626 a, 626 b it generates green to redlight which in combination with unconverted blue excitation lightgenerates a combined light emission 660 of a third color temperaturethat is between the first and second color temperatures.

Since the photoluminescence layer 642 covers the photoluminescence layer640 this can improve mixing of light and improve color uniformity ofemitted light.

It will be appreciated that depending on the relative proportion oflight generated by the first and second LED arrays the color temperatureof the LED-filament can be tuned between the first and second colortemperatures. Additionally, particles of a light scattering material canbe combined with the photoluminescence materials to reduce the quantityof photoluminescence material required to generate a given emissionproduct color.

The LED-filament of FIG. 6 can be manufactured by first depositing thephotoluminescence layer 640 onto the first LED array 626 a and thendepositing the photoluminescence layer 642 on the firstphotoluminescence layer 640 and on the second LED array 626 b. Thephotoluminescence layer 658 can then be deposited on to cover the backface 628 of the substrate 624. As illustrated, the photoluminescencelayers 640, 642, 644 can have a cross section that is generallysemi-circular in profile. A particular advantage of the structure of thephotoluminescence arrangement of FIG. 6 is its ease of fabrication sinceit comprises only two layers which also makes it structurally robust.

The LED-filament of FIG. 6 can find particular utility where it isrequired that the first and second LED arrays generate light of similar(<1500K difference), especially lower (warmer), color temperatures, e.g.“warm white” to “amber” and cooler “warm white” thereby enabling theLED-filament to be tunable between “amber” and “warm white”. Such arange of color tuning is highly desirable for “warm dimming”. Forexample, the photoluminescence compositions of the photoluminescencelayers 640 and 642 can be configured such that the photoluminescencelayers 640, 642 when excited by blue light from the first array of LEDsgenerates light of a first color temperature, for example “warm white”(e.g. CCT 1500K to 2500K), when excited by blue light from the secondLED array generates light of a second, higher, color temperature, forexample a cooler “warm white” (e.g. CCT 3000K to 4000K) and when excitedequally by blue light from the first and second LED arrays generateslight of an intermediate color temperature, for example 2700K.

While in each of the foregoing embodiments the first and second arraysof LED chips are mounted on the same face of the substrate for ease offabrication and to reduce costs, it is contemplated in other embodimentsto mount the first and second arrays of LED chips on opposite faces ofthe substrate.

In various embodiments of the invention, and to reduce photoluminescencematerial usage, the LED-filament can further comprise particles of alight scattering material such as for example particles of zinc oxide(ZnO), titanium dioxide (TiO₂) barium sulfate (BaSO₄), magnesium oxide(MgO), silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), zirconium dioxide(ZrO₂) or mixtures thereof. The particles of light scattering materialcan be provided as a mixture with any of the photoluminescence materialsand/or in a separate layer in contact with a photoluminescence materiallayer.

The inclusion of particles of a light scattering material with thephotoluminescence material increases the number of collisions of LEDgenerated excitation light with particles of the photoluminescencematerial enhancing photoluminescence light generation which decreasesthe amount of photoluminescence material usage. It is believed that onaverage as little as1 in 10,000 interactions of a photon with aphotoluminescence material results in absorption and generation ofphotoluminescence light. The majority, about 99.99%, of interactions ofphotons with a photoluminescence material particle result in scatteringof the photon. Since the inclusion of the light scattering materialsincreases the number of collisions this increases the probability ofphotoluminescence light generation, which decreases the amount ofphotoluminescence material usage to generate a selected emissionintensity.

Green to Yellow Photoluminescence Materials

In this patent specification, a broadband green to yellowphotoluminescence material refers to a material which generates lighthaving a peak emission wavelength (λ_(pe)) in a range ˜520 nm to ˜560nm, that is in the yellow/green to green region of the visible spectrum.Preferably, the green photoluminescence material has a broad emissioncharacteristic and preferably has a FWHM (Full Width at Half Maximum) ofbetween about 50 nm and about 120 nm. The green photoluminescencematerial can comprise any photoluminescence material, such as forexample, garnet-based inorganic phosphor materials, silicate phosphormaterials and oxynitride phosphor materials. Examples of suitable greenphosphors are given in TABLE 1.

In some embodiments, the green photoluminescence materials comprises acerium-activated yttrium aluminum garnet phosphor of general compositionY₃(Al_(1-y)Ga_(y))₅O₁₂:Ce (YAG) where 0<y<1 having a peak emissionwavelength of in a range 520 nm to 543 nm and a FWHM of ˜120 nm. In thispatent specification, the notation YAG# represents the phosphortype—YAG—based phosphors—followed by the peak emission wavelength innanometers (#). For example, YAG535 denotes a YAG phosphor with a peakemission wavelength of 535 nm. The green photoluminescence material maycomprise a cerium-activated yttrium aluminum garnet phosphor of generalcomposition (Y,Ba)₃(Al,Ga)₅O₁₂:Ce (YAG). In some embodiments, the greenphotoluminescence material can comprise an aluminate (LuAG) phosphor ofgeneral composition Lu₃Al₅O₁₂:Ce (GAL) having a peak emission wavelengthof 516 nm to 560 nm and a FWHM of ˜120 nm. In this patent specification,the notation GAL# represents the phosphor type (GAL)—LuAG—basedphosphors—followed by the peak emission wavelength in nanometers (#).For example, GAL520 denotes a GAL phosphor with a peak emissionwavelength of 520 nm. Suitable green phosphors are given in TABLE 1.

Examples of green silicate phosphors include europium activatedortho-silicate phosphors of general composition (Ba, Sr)₂SiO₄:Eu such asfor example G, EG, Y and EY series of phosphors from IntematixCorporation, Fremont Calif., USA which have a peak emission wavelengthin a range 507 nm to 570 nm and a FWHM of ˜70 nm to ˜80 nm. Suitablegreen phosphors are given in TABLE 1.

In some embodiments, the green phosphor can comprise a green-emittingoxynitride phosphor as taught in U.S. Pat. No. 8,679,367 entitled “GreenEmitting (Oxy) Nitride-Based Phosphors and Light Emitting Devices Usingthe Same” which is hereby incorporated in its entirety. Such agreen-emitting oxynitride (ON) phosphor can have a general compositionEu²⁺:M²⁺Si₄AlO_(x)N_((7-2 x/3)) where 0.1≤x≤1.0 and M²⁺ is one or moredivalent metal selected from the group consisting of Mg, Ca, Sr, Ba, andZn. In this patent specification, the notation ON# represents thephosphor type (oxynitride) followed by the peak emission wavelength(λ_(pe)) in nanometers (#). For example ON495 denotes a green oxynitridephosphor with a peak emission wavelength of 495 nm.

TABLE 1 Example broadband green photoluminescence materials WavelengthPhosphor General Composition λ_(p) (nm) YAGY_(3-x)(Al_(1-y)Ga_(y))₅O₁₂:Ce_(x) 0.01 < x < 0.2 & 0 < y < 2.5 520-550(YAG#) GNYAG (Y, Ba)_(3-x)(Al_(1-y)Ga_(y))₅O₁₂:Ce_(x) 0.01 < x < 0.2 & 0< y < 2.5 520-550 (YAG#) LuAG Lu_(3-x)(Al_(1-y)M_(y))₅O₁₂:Ce_(x) 0.01 <x < 0.2 & 0 < y < 1.5M = Mg, 500-550 (GAL#) Ca, Sr, Ba, Ga, LuAGLu_(3-x)(Al_(1-y)Ga_(y))₅O₁₂:Ce_(x) 0.01 < x < 0.2 & 0 < y < 1.5 500-550(GAL#) Silicate A₂SiO₄:Eu A = Mg, Ca, Sr, Ba 500-550 Silicate(Sr_(1-x)Ba_(x))₂SiO₄:Eu 0.3 < x < 0.9 500-550 OxynitrideEu²⁺:M²⁺Si₄AlO_(x)N_((7-2x/3)) M²⁺ = Mg, Ca, Sr, Ba, Zn 500-550 (ON#)0.1 ≤ x ≤ 1.0

Orange to Red Photoluminescence Materials

Narrowband Red Photoluminescence Materials

In this patent specification, a narrowband red photoluminescencematerial refers to a photoluminescence material which, in response tostimulation by excitation light, generates light having a peak emissionwavelength in a range 610 nm to 655 nm; that is light in the red regionof the visible spectrum and which has a narrow emission characteristicwith a full width at half maximum (FWHM) emission intensity of betweenabout 5 nm and about 50 nm (less than about 50 nm). As described above,the narrowband red photoluminescence can comprise a manganese-activatedfluoride red photoluminescence material that is disposed on and coversthe front face of the substrate on which the LED chips are mounted. Anexample of a narrowband red manganese-activated fluoridephotoluminescence material is manganese-activated potassiumhexafluorosilicate phosphor (KSF)—K₂SiF₆:Mn⁴⁺ (KSF). Othermanganese-activated phosphors can include: K₂GeF₆:Mn⁴⁺ (KGF) andK₂TiF₆:Mn⁴⁻ (KTF).

Broadband Red Photoluminescence Materials

In this patent specification, a broadband red photoluminescence material(also referred to as a non-manganese-activated fluoride redphotoluminescence material) refers to a photoluminescence materialwhich, in response to stimulation by excitation light, generates lighthaving a peak emission wavelength in a range 600 nm to 640 nm; that islight in the orange to red region of the visible spectrum and which hasa broad emission characteristic with a full width at half maximum (FWHM)emission intensity of greater than about 50 nm. As described above, thebroadband red photoluminescence can comprise rare-earth activated redphotoluminescence materials. A broadband red photoluminescence material(non-manganese-activated fluoride red photoluminescence material)denotes a red photoluminescence material whose crystal structure isother than that of a narrowband red photoluminescence material(manganese-activated fluoride photoluminescence material), such as forexample rare-earth-activated red photoluminescence materials and cancomprise any such red photoluminescence material that is excitable byblue light and operable to emit light with a peak emission wavelengthλ_(p) in a range about 600 nm to about 640 nm. Rare-earth-activated redphotoluminescence material can include, for example, a europiumactivated silicon nitride-based phosphor, α-SiAlON, Group IIA/IIBselenide sulfide-based phosphor or silicate-based phosphors. Examples ofred phosphors are given in TABLE 2.

In some embodiments, the europium activated silicon nitride-basedphosphor comprises a Calcium Aluminum Silicon Nitride phosphor (CASN) ofgeneral formula CaAlSiN₃:Eu²⁻. The CASN phosphor can be doped with otherelements such as strontium (Sr), general formula (Sr,Ca)AlSiN₃:Eu²⁺. Inthis patent specification, the notation CASN# represents the phosphortype (CASN) followed by the peak emission wavelength (λ_(pe)) innanometers (#). For example, CASN625 denotes a red CASN phosphor with apeak emission wavelength of 625 nm.

In an embodiment, the rare-earth-activated red phosphor can comprise ared-emitting phosphor as taught in U.S. Pat. No. 8,597,545 entitled“Red-Emitting Nitride-Based Calcium-Stabilized Phosphors” which ishereby incorporated in its entirety. Such a red emitting phosphorcomprises a nitride-based composition represented by the chemicalformula M_(a)Sr_(b)Si_(c)Al_(d)N_(e)Eu_(f), wherein: M is Ca, and0.1≤a≤0.4; 1.5<b<2.5; 4.0≤c≤5.0; 0.1≤d≤0.15; 7.5<e<8.5; and 0<f<0.1;wherein a+b+f>2+d/v and v is the valence of M.

Alternatively, the rare-earth-activated red phosphor can comprise a redemitting nitride-based phosphor as taught in U.S. Pat. No. 8,663,502entitled “Red-Emitting Nitride-Based Phosphors” which is herebyincorporated in its entirety. Such a red emitting phosphor comprising anitride-based composition represented by the chemical formulaM_((x/v))M′₂Si_(5-x)Al_(x)N₈:RE, wherein: M is at least one monovalent,divalent or trivalent metal with valence v; M′ is at least one of Mg,Ca, Sr, Ba, and Zn; and RE is at least one of Eu, Ce, Tb, Pr, and Mn;wherein x satisfies 0.1≤x<0.4, and wherein said red-emitting phosphorhas the general crystalline structure of M′₂Si₅N₈:RE, Al substitutes forSi within said general crystalline structure, and M is located withinsaid general crystalline structure substantially at the interstitialsites.

Rare-earth-activated red phosphors can also include Group IIA/BBselenide sulfide-based phosphors. A first example of a Group IIA/IIBselenide sulfide-based phosphor material has a compositionMSe_(1-x)S_(x):Eu, wherein M is at least one of Mg, Ca, Sr, Ba and Znand 0<x<1.0. A particular example of this phosphor material is CSSphosphor (CaSe_(1-x)S_(x):Eu). Details of CSS phosphors are provided inco-pending United States patent application Publication NumberUS2017/0145309 filed 30 Sep. 2016, which is hereby incorporated byreference in its entirety. The CSS red phosphors described in UnitedStates patent publication US2017/0145309 can be used in the presentinvention. The emission peak wavelength of the CSS phosphor can be tunedfrom 600 nm to 650 nm by altering the S/Se ratio in the composition andexhibits a narrowband red emission spectrum with FWHM in the range ˜48nm to ˜60 nm (longer peak emission wavelength typically has a largerFWHM value). In this patent specification, the notation CSS# representsthe phosphor type (CSS) followed by the peak emission wavelength innanometers (#). For example, CSS615 denotes a CSS phosphor with a peakemission wavelength of 615 nm.

In some embodiments, the rare-earth-activated red phosphor can comprisean orange-emitting silicate-based phosphor as taught in U.S. Pat. No.7,655,156 entitled “Silicate-Based Orange Phosphors” which is herebyincorporated in its entirety. Such an orange-emitting silicate-basedphosphor can have a general composition (Sr_(1-x)M_(x))_(y)Eu_(z)SiO₅where 0<x≤0.5, 2.6≤y≤3.3, 0.001≤z≤0.5 and M is one or more divalentmetal selected from the group consisting of Ba, Mg, Ca, and Zn. In thispatent specification, the notation O# represents the phosphor type(orange silicate) followed by the peak emission wavelength (λ_(pe)) innanometers (#). For example, O600 denotes an orange silicate phosphorwith a peak emission wavelength of 600 nm.

TABLE 2 Example broadband red photoluminescence materials Wave- Generallength Phosphor Composition λ_(p) (nm) CASN (Ca_(1-x)Sr_(x))AlSiN₃:Eu0.5 < x ≤ 1 600-650 (CASN#) 258 nitride Ba_(2-x)Sr_(x)Si₅N₈:Eu 0 ≤ x ≤ 2580-650 Group IIA/BB Selenide Sulfide MSe_(1-x)S_(x):Eu M = Mg, Ca, Sr,Ba, Zn 600-650 (CSS#) 0 < x < 1.0 CSS CaSe_(1-x)S_(x):Eu 0 < x < 1.0600-650 (CSS#) Silicate (Sr_(1-x)M_(x))_(y)Eu_(z)SiO₅ M = Ba, Mg, Ca, Zn565-650 (O#) 0 < x ≤ 0.5 2.6 ≤ y ≤ 3.3 0.001 ≤ z ≤ 0.5

FIG. 7A is a schematic of a first driver circuit arrangement 762 for usewith the color temperature tunable LED-filament of FIGS. 3 to 6 enablingindependent power control to the first and second LED arrays. In thecircuit arrangement 762 the first and second LED arrays 726 a, 726 beach comprise a string of serially electrically connected LEDs. Asdescribed above, the first LED array 726 a in conjunction with theirrespective photoluminescence arrangement generate light of a first colortemperature (warmer) while the second LED array 726 b in conjunctionwith their respective photoluminescence arrangement generate light of asecond higher color temperature that is cooler. As indicated, the firstand second LED arrays can have the same numbers of LEDs. Sinceelectrical power (e.g. current I_(a) and I_(b)) applied to each of theLED arrays is independently controllable, the color of light generatedby the LED-filament can be tuned by controlling the relative power(current) flowing through each LED string. It will be appreciated thatthe LED-filament can be operated to exhibit “warm diming”, that is thecolor temperature of light generated by the LED-filament decreases (i.e.becomes warmer) as power to the LED-filament is decreased (“dimmed”), bythe driver circuitry used to generate appropriate drive currents I_(a)and I_(b). Such driver circuitry can be housed within the connector/base102 (FIG. 1A) of the LED-filament bulb.

FIG. 7B is a schematic of a second driver circuit arrangement 764 foruse with the color temperature tunable LED-filament of FIGS. 3 to 6 thatprovides “warm dimming” without the use of additional circuitry. In thecircuit arrangement 764 the first LED array 726 a comprises a string ofserially electrically connected LEDs that is connected in series with aresistive component (resistor) 766 and the second LED array 726 acomprises a string of serially electrically connected LEDs. The firstLED array 726 a/resistor 766 and the second LED array 726 b areelectrically connected in parallel. As indicated in FIG. 7B, the numberof LEDs in the first LED array 726 a may be fewer than the number ofLEDs in the second LED array 726 b. The resistor 766 is typicallymounted on the substrate of the LED-filament. As described above, thefirst LED array 726 a in conjunction with its photoluminescencearrangement generate light of a first color temperature (warmer) whilethe second LED array 726 b in conjunction with its photoluminescencearrangement generate light of a second higher color temperature(cooler). The effect of having different numbers of LEDs in each stringand the addition of the resistor 766 in the first string is that therelative proportion of the current I applied to the LED-filament flowingthrough the first and second LEDs (I_(a) and I_(b) respectively) varieswith applied power (current I). At low powers (current), i.e.corresponding to LED-filament being dimmed, a greater or even majority,proportion of the current I flows through the first LED array 926 a andthe LED-filament thus produces light that is predominantly of the warmercolor. As the current I is increased the voltage drop across resistor766 increases which reduces the proportion of current I_(a) flowingthrough the first LED array relative to the proportion I_(b) flowingthrough the second LED array and the LED-filament produces light whichbecomes increasing cooler (higher color temperature). By appropriateconfiguration of the relative difference in numbers of LEDs in the firstand second arrays and the resistance of the resistor 766, theLED-filament can be configured such that at full power, a greater, ormajority, proportion of the current I_(b) passes through the second LEDarray 726 b and the LED-filament thus produces light that ispredominantly of the cooler color. A particular benefit of thearrangement of FIG. 9B is that an LED-filament bulb utilizing suchLED-filaments can be operated using a conventional dimmer and exhibit“warm dimming”.

FIGS. 8A to 8C are test data for a first “warm dimming” LED-filament inaccordance with FIG. 7B and respectively show: (a) current/voltage(I_(x)/V) characteristic of the first and second LED arrays of theLED-filament, (b) currents I_(a) and I_(b) (mA) flowing through thefirst and second LED arrays as a function of filament power (W), and (c)current ratio I_(x)/I (%) for currents I_(a) and I_(b) flowing throughthe first and second LED arrays as a function of filament power (W). Inthis example, the first LED array 726 a comprises twenty four LED chipsand a 400Ω resistor 766 and the second LED array 726 b comprises twentysix LED chips. Referring to the FIG. 8A, it is to be noted that due tothe presence of the resistor 766 the current/voltage (I_(a)/V)characteristic of the first LED array (warmer) has a generally linearcharacteristic. This is to be contrasted with the current/voltage(I_(b)/V) characteristic of the second LED array (cooler) which, aswould be expected, exhibits a typical exponentially increasing LEDcharacteristic. It is as a result of these different characteristicsthat enables “warm dimming”. Referring to FIG. 8B, it is to be notedthat as the current I applied to the LED-filament increases the currentI_(a) flowing through the first LED array increases and flattens out atmaximum (saturation) value of about 15 mA, while the current I_(b)flowing through the second LED array continues to increase. As can beseen from FIG. 8C in terms of current, is that at low powers (0.05 W to0.3 W-5% to 10% maximum power), corresponding to dimming, virtually 100%of the current I flows through the first LED array and the LED-filamentgenerates light of a warm color. As power is increased to theLED-filament, the current I_(a) flowing through the first LED arraydecreases while the current I_(b) flowing through the second LED arrayincreases. At maximum power (about 3 W), about 30% of the current Iflows through the first LED array and a majority (70%) of the current Iflows through the second LED array. As a result at maximum power, theLED-filament generates light of a cooler (i.e. higher CCT) color.

FIGS. 9A to 9C are test data for a second “warm dimming” LED-filament inaccordance with FIG. 7B and respectively show: (a) current/voltage(I_(x)/V) characteristic of the first and second LED arrays of theLED-filament, (b) currents I_(a) and I_(b) (mA) flowing through thefirst and second LED arrays as a function of filament power (W), and (c)current ratio I_(x)/I (%) for currents I_(a) and I_(b) flowing throughthe first and second LED arrays as a function of filament power (W). Inthis example, the first LED array 726 a comprises twenty four LED chipsand a 200Ω resistor and the second LED array 726 b comprises twenty sixLED chips. These figures indicate how the relative current flowingthrough the and second arrays of LEDs at maximum power (about 4 W) canbe selected using the resistance of the resistor 766 and thereby selectthe color of light generated by the LED-filament at maximum power. Ascan be seen from FIG. 9A, the effect of reducing the resistance ofresistor 766 is an increase in the slope of the current/voltage(I_(a)/V) characteristic of the first LED array. This results in firstLED array now having a maximum (saturation) value of about 30 mA at fullpower. As a result, at full power the LED-filament will generate lightof CCT that is between warmer and cooler colors.

FIGS. 10A and 10B are measured test data for a tunable LED-filament withthe photoluminescence arrangement of FIG. 6 comprising a first LED arrayfor generating light of CCT 2700K and a second LED array for generatinglight of CCT 5000K. FIG. 10A shows intensity spectra, normalizedintensity I versus wavelength (nm) for (i) the LED-filament foroperation of the first LED array only (2700K-thick solid line), (ii)black-body-curve (bbc)—Planckian spectrum (2700K-dashed line) for a CCTof 2700K, (iii) the LED-filament for operation of the second LED arrayonly (5000K-dotted line), and (iv) CIE Standard Illuminant D50 (CIED50-thin solid line) for a CCT of 5000K. FIG. 10B shows measured CIE1931 chromaticity data CIE x, CIE y for (i) the LED-filament foroperation of the first LED array only (circular dots), (ii) theLED-filament for operation of the second LED array only (triangularpoints), (iii) locus of chromaticity (color temperature) change (solidline) for operation of both LED arrays, and (iv) the black body locusfor reference (dashed line). FIG. 10B further includes chromaticitycoordinates (CIE x and CIE y) for ANSI standard coordinates for 2700K(0.458, 0.416) and 5000K (0.345, 0.355) and their respective 3 and 5step MacAdam ellipses. The tunable LED-filament comprises a ceramicsubstrate of dimensions 30 mm by 3.5 mm having first and second LEDarrays each comprising 25 LED chips (dies).

As can be seen from FIG. 10A, when electrical power is applied to onlythe first LED array (i.e. I=I_(a) & I_(b)=0), the LED-filament produceslight of CCT 2700K whose spectra (thick solid line) closely resemblesthe black body curve (dashed line) of the same color temperature. Itwill be further seen that when electrical power is applied to only thesecond LED array (i.e. I=I_(b) & I_(a)=0) the LED-filament produceslight of CCT 5000K whose spectra (dotted line) closely resembles the CIEStandard Illuminant D50 (thin solid line). In terms of chromaticity, itwill be seen from FIG. 10B that when electrical power is applied only tothe first LED array (i.e. I=I_(a) & I_(b)=0) the LED-filament produceslight (circular dots) of CCT 2700K whose chromaticity is very close tothe black body locus (dashed line) and is within the 3 step MacAdamellipse of the ANSI standard coordinates for 2700K. Moreover, whenelectrical power is applied only to the second LED array (i.e. I=I_(b) &I_(a)=0) the LED-filament produces light (triangular points) of CCT5000K whose chromaticity is very close to the black body locus (dashedline) and is within the step MacAdam ellipse of the ANSI standardcoordinates for 5000K. The solid line in FIG. 10B is the locus of thechromaticity of light generated by the LED-filament as the proportion ofpower (i.e. I_(a), I_(b)) applied to the first and second LED arrays isvaried, for example, from I=I_(a) & I_(b)=0 to I=I_(b) & I_(a)=0. Thelocus, which is substantially a straight line, connects the chromaticitycoordinates for the color temperatures 2700K (I=I_(a) & I_(b)=0) and5000l K (i.e. I=I_(b) & I_(a)=0) and demonstrates that LED-filaments inaccordance with the invention are capable of producing light of anycolor temperature from 2700K to 5000K by varying the proportion of I_(a)and I_(b).

As used in this document, both in the description and in the claims, andas customarily used in the art, the words “substantially,”“approximately,” and similar terms of approximation are used to accountfor manufacturing tolerances, manufacturing variations, andmanufacturing imprecisions that are inescapable parts of fabricating andoperating any mechanism or structure in the physical world.

While the invention has been described in detail, it will be apparent toone skilled in the art that various changes and modifications can bemade and equivalents employed, without departing from the presentinvention. It is to be understood that the invention is not limited tothe details of construction, the arrangements of components, and/or themethod set forth in the above description or illustrated in thedrawings. Statements in the abstract of this document, and any summarystatements in this document, are merely exemplary; they are not, andcannot be interpreted as, limiting the scope of the claims; the purposeof the abstract is to enable the U.S. Patent and Trademark Office, aswell as readers who are not familiar with patent or legal terms orphraseology, to determine quickly from a cursory inspection the natureand essence of the technical disclosure of the application. Further, thefigures are merely exemplary and not limiting. Topical headings andsubheadings are for the convenience of the reader only. They should notand cannot be construed to have any substantive significance, meaning orinterpretation, and should not and cannot be deemed to indicate that allof the information relating to any particular topic is to be found underor limited to any particular heading or subheading. Therefore, theinvention is not to be restricted or limited except in accordance withthe following claims and their legal equivalents.

REFERENCE NUMERALS

-   #00 LED-filament lamp-   #02 Connector base-   #04 Envelope-   #06 LED-filament support-   #08 LED-filament-   #10 Volume-   #12 Lamp axis-   #14 First electrical contact-   #16 First wire-   #18 Second electrical contact-   #20 Second wire-   #22 Base portion of LED-filament support-   #24 Light-transmissive substrate-   #26 LED chip-   #28 Front face of substrate-   #30 Bond wire-   #32 Bond wire-   #34 Back face of substrate-   #36 Photoluminescence wavelength conversion coating on front face of    substrate-   #38 Photoluminescence wavelength conversion coating on back face of    substrate-   #40 Photoluminescence layer (front face)-   #42 Photoluminescence layer (front face)-   #44 Photoluminescence layer (back face)-   #46 Photoluminescence layer (back face)-   #48 First color temperature light-   #50 Second color temperature light-   #52 Diffusing layer-   #54 Photoluminescence layer (front face)-   #56 Photoluminescence layer (front face)-   #58 Photoluminescence layer (back face)-   #60 Third color temperature light Photoluminescence layer (front    face)-   #62 First driver circuit arrangement-   #64 Second driver circuit arrangement-   #66 Resistive load

1-19. (canceled)
 20. An LED-filament comprising: a light-transmissivesubstrate; a first array of LED chips on a front face of the substrate;a second array of LED chips on the front face of the substrate; a firstlayer having a first photoluminescence material, wherein the first layeris continuous and covers all LED chips of the first array; and a secondlayer having a second photoluminescence material that covers all LEDchips of the second array and the first layer of photoluminescencematerial.
 21. The LED-filament of claim 20, wherein the first array ofLED chips, the first photoluminescence material and the secondphotoluminescence material generates light of a first color temperatureof 1500K to 3500K and the second array of LED chips and the secondphotoluminescence material generates light of a second color temperatureof 3500K to 7500K.
 22. The LED-filament of claim 20, wherein the firstphotoluminescence material comprises a manganese-activated fluoridenarrowband red photoluminescence material.
 23. The LED-filament of claim22, wherein the narrowband red photoluminescence material is at leastone of: K₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺, and K₂TiF₆:Mn⁴⁺.
 24. The LED-filamentof claim 20, comprising a third layer having a third photoluminescencematerial disposed on the back face of the substrate.
 25. TheLED-filament of claim 20, wherein at least one of the first and secondlayers has a cross section that is generally semi-circular in profile.26. The LED-filament of claim 20, wherein the first and second arrays ofLED chips each respectively comprise a linear array of LED chips andwherein the respective linear arrays are arranged in parallel on thesubstrate.
 27. An LED-filament comprising: a light-transmissivesubstrate; a first array of LED chips on a front face of the substrate;a second array of LED chips on the front face of the substrate; a firstmaterial comprising at least one photoluminescence material covering thefirst array of LED chips; and a second material comprising at least onephotoluminescence material covering the second array of LED chips;wherein the first array of LED chips and the first material generate amajority of light of a first color temperature and the second array ofLED chips and the second material generate a majority of light of asecond color temperature and wherein at least one of the first andsecond materials comprises a manganese-activated fluoride narrowband redphotoluminescence material.
 28. The LED-filament of claim 27, whereinthe narrowband red photoluminescence material is at least one of:K₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺, and K₂TiF₆:Mn⁴⁺.
 29. The LED-filament of claim27, wherein the first material comprises a layer comprising a majoritynarrowband red photoluminescence material that is deposited directly onthe first array of LED chips.
 30. The LED-filament of claim 27, whereinthe first color temperature is 1500K to 3500K and the second colortemperature is 3500K to 7500K, or the first color temperature is 1800Kto 2500K and the second color temperature is 3000K to 4000K.
 31. TheLED-filament of claim 27, further comprising a diffusing layer havingparticles of light scattering material covering the first and secondphotoluminescence materials.
 32. The LED-filament of claim 27, furthercomprising a third material comprising at least one photoluminescencematerial covering the first material such that the first array of LEDchips, the first material and third material generate light of the firstcolor temperature.
 33. The LED-filament of claim 32, further comprisinga fourth material comprising at least one photoluminescence materialcovering the second material such that the second array of LED chips,the second material and fourth material generate light of the secondcolor temperature.
 34. The LED-filament of claim 27, wherein the secondmaterial covers both the first material and the second array of LEDchips such that the first array of LED chips, the first material andsecond material generate light of the first color temperature.
 35. Awarm dimming LED-filament comprising: a substrate; a first array of LEDchips on the substrate; a second array of LED chips on the substrate; afirst photoluminescence material; and a second photoluminescencematerial; wherein the first array and first material generate light of afirst color temperature and the second array and second materialgenerate light of a second cooler color temperature; and wherein thefirst array comprises serially connected LED chips connected in serieswith a resistive component and the second array comprises seriallyconnected LED chips; and wherein the first and second arrays areelectrically connected in parallel.
 36. The LED-filament of claim 35,wherein the number of LED chips in the first and second arrays and theresistive component are selected such that the current/voltagecharacteristic (I-V) of the first array increases predominantly linearlywith voltage and the current/voltage characteristic (I-V) of the secondarray increases generally exponentially with voltage.
 37. TheLED-filament of claim 35, wherein the number of LED chips in the firstand second arrays and the resistive component are selected such that atmaximum power current flows through the first and second arrays and theLED-filament produces light which is a combination of the first andsecond color temperatures and at minimum power a majority proportion ofcurrent flows through the first array and the filament produces light ofpredominantly the first color temperature.
 38. The LED-filament of claim37, wherein at maximum power at least 50% of the current flows throughthe second array.
 39. The LED-filament of claim 35, wherein the firstarray comprises fewer LED chips than the second array.